It really depends on what kind of approach nVIDIA took. If the rumours are correct 55nm G200's have already been taped out and has probably already entered the volume production stage to go retail in next month.
IHMO the move to 55nm has probably been made half a year ago. With rumours of it being taped out and soon to be released, this is only possible if such a decision was made roughly 5~6months ago. (Since even the easiest of transitions to a smaller process takes months). I think this makes perfect sense seeing as that would coincide with them missing their next gen launch around nov~dec timeframe in 2007 because they went with the 55nm route and we all know the reason why.
What does this mean? i think the initial wave of the GTX200 series based on 65nm process was just a strategic move to see what kind of product AMD had in store. Since performance figures are already out, i think nVIDIA can easily counter those products with its 55nm process based products. The GTX260 and 280 severely lack in the shader clock frequency. I can see this part of the architecture being healthy boosted to around 1400~1500MHz.
Im not sure what kind of approach nVIDIA took when transitioning its G200 GPU to the 55nm process. It could be a simple die shrink, i.e the same architecture (since theres no need to change the layout of the architecture when going to a half node process). It could be something along the lines of G70 to G71 i.e to making the G200 much more economical than a simple die shrink (removal of the unnecessary fat/transistors, re-structuring/layout of its architecture to decrease GPU die size since G200 isnt densely laid out like RV770). Or the last one is highly unlikely but some major reworking has been done to provide new features such as DX10.1, or the architecture is based around a 256bit+GDDR5 combination. Personally the first two sounds more likely.